Additive manufacturing (AM), also known as solid freeform fabrication or 3D printing, refers to any manufacturing process where three-dimensional objects are built up from raw material (generally powders, wires, liquids, suspensions, or molten solids) in a series of two-dimensional layers or cross-sections. In contrast, traditional machining techniques involve subtractive processes and produce objects that are cut out of a stock material such as a block of wood or metal.
A variety of additive processes can be used in additive manufacturing. The various processes differ in the way layers are deposited to create the finished objects and in the materials that are compatible for use in each process. Some methods melt or soften material to produce layers, e.g., selective laser melting (SLM) or direct metal laser sintering (DMLS), selective laser sintering (SLS), fused deposition modeling (FDM), while others cure liquid materials using different technologies, e.g. stereolithography (SLA).
Sintering is a process of fusing small grains (or particles), e.g., powders, to create objects. Sintering usually involves heating a powder. When a powdered material is heated to a sufficient temperature (typically lower than the melting point) in a sintering process, the atoms in the powder particles diffuse across the boundaries of the particles, fusing the particles together to form a solid piece. In contrast to melting, the powder used in sintering need not reach a liquid phase. As the sintering temperature does not have to reach the melting point of the material, sintering is often used for materials with high melting points such as tungsten and molybdenum.
Both sintering and melting can be used in additive manufacturing. The material being used determines which process occurs. An amorphous solid, such as acrylonitrile butadiene styrene (ABS), is actually a supercooled viscous liquid, and does not actually melt; as melting involves a phase transition from a solid to a liquid state. Thus, selective laser sintering (SLS) is the relevant process for ABS, while selective laser melting (SLM) is used for crystalline and semi-crystalline materials such as nylon and metals, which have a discrete melting/freezing temperature and undergo melting during the SLM process.
Conventional powder dispensing systems that use a laser beam or electron beam as the energy source for sintering or melting a powdered material typically direct the beam on a selected point in a layer of the powdered material and selectively raster scan the beam to locations across the layer. Once all the selected locations on the first layer are sintered or melted, the platform which supports the powder is moved downwards and a new layer of powdered material is deposited on top of the completed layer. The process is repeated layer by layer until the desired object is produced.